Seismic behavior of steel tube confined RC columns under compression-bending-torsion combined load

In order to study the mechanical behavior of steel tube confined reinforced concrete (STRC) columns under combined compression-bending-torsion, an experimental investigation was conducted. The load-deformation hysteretic curves and the strain distribution law of steel tubes under pure torsion, bending-torsion and compression-bending-torsion loads were obtained. Results show that a 'pinch' effect appeared in the load-deformation hysteretic curves of STRC under pure torsion and bending-torsion loads, however, this was not noticeable for specimens under compression-bending-torsion load. When the axial compression ratio was relatively small, STRC columns under compression-bending-torsion load showed higher bending capacity and torsion capacity than that under bending-torsion load, as well as good energy dissipation capacity. Furthermore, the strain of the steel tube increased linearly with the load, and the corresponding stress was always below the yield strength. Based on the test results, the enhancement of the bending and torsion capacities of reinforced concrete columns caused by steel tubes, and the reduction of the bending capacity of STRC columns caused by torque were analyzed.